The present invention relates to a load-clamping system for gripping a load between a selectively closeable and openable pair of load-engaging surfaces and, more particularly, to such a clamp which automatically increases the grip of the load-engaging surfaces in response to the sensing of any slippage between the load and the surfaces.
Numerous types of systems for automatically varying gripping force have been proposed in the past for load handling clamps, whether of the electric-powered or fluid-powered type. These previous systems can generally be categorized as follows:
(1) systems which sense the existence of slippage and respond automatically by gradually increasing the gripping force on the load by fixed force increments until the sensed slippage stops; PA1 (2) systems which gradually vary the gripping force in proportion either to the sensed weight or to the resistance to gripping of the load, without regard to whether or not slippage is actually occurring; and PA1 (3) systems which perform a combination of (1) and (2). PA1 European Patent Publication No. 0443998A1, Aug. 28, 1991; PA1 Japanese Patent Publication No. 48-35559, May 25, 1973; PA1 Japanese Patent Publication No. 48-36849, May 31, 1973; PA1 Japanese Patent Publication, No. 52-33253, Mar. 14, 1977; PA1 Japanese Patent Publication, No. 53-44744, Dec. 1, 1978; PA1 M. Ueda et al., "Sensors in Systems Necessary for Industrial Robots in the Near Future," Dept of Electrical Engineering, Nagoya University, Nagoya, Japan, November 1974, pp. 79-88; PA1 R. Tomovic et al., "Multi-Functional Terminal Device with Adaptive Grasping Force," Automatica, Pergamon Press, Vol. 11, 1975, pp. 567-570; PA1 J. Kallhammer, "Incipient Slip Detection Using a Tactile Sensor," Dept. of Electrical Engineering, Duke University, Durham, N.C., 1985; PA1 D. Brown et al., "Design and Implementation of a Computer-Controlled Sensor-Equipped Robot End Effector," North-Holland Computers in Industry, Vol. 11, 1988, pp. 119-133. PA1 P. Merin, "How Mechatronic Engineering Led to `Intelligent` Paper Roll Clamps," TAPPI Proceedings, 1993 Finishing and Converting Conference, 1993, pp. 63-70. PA1 German Patent Publication, No. 2636473, Feb. 23, 1978; PA1 German Patent Publication, No. 3245715, Sep. 1, 1983; PA1 U.S. Pat. No. 4,783,106, issued Nov. 8, 1988.
The first-mentioned category, wherein gripping force is increased gradually by fixed increments automatically in response to sensed slippage until the slippage stops, has been the most prevalent. The primary objective of such systems is to apply the minimum gripping force necessary to prevent slippage, to avoid undue deformation or other damage to the load. The problem, however, is that such systems can fail to achieve this objective because mere slippage detection, followed by gradual increases in gripping force until the slippage finally stops, provides an increase in grip which can be much too slow to prevent the load from slipping considerably and gaining significant slipping momentum, depending upon the application. In such case, an excessively high increase in gripping force, accompanied by resultant excessive load deformation and surface damage to the load, is eventually required to counteract the slipping momentum. Systems operating on this principle are disclosed in the following publications:
Systems of the second category mentioned above gradually increase gripping force automatically in response to factors other than the concurrent sensing of slippage, although some of them sense the coefficient of friction between the load and the load-engaging surfaces by preliminary slip detection prior to lifting the load and later use this coefficient as a factor in quantifying the gripping force variations. None of these systems, however, can anticipate slippage caused by shock or other dynamic loading accurately enough to prevent it. Moreover, because they do not respond to the concurrent sensing of slippage after the load has been lifted, they cannot correct it. Examples of systems of this type are disclosed in the following publications:
Systems of the third category mentioned above, which gradually increase the gripping force by fixed increments automatically in response both to the existence of slippage and to the sensed weight of the load, nevertheless cannot anticipate slippage caused by shock or other dynamic loading accurately enough to prevent it. When slippage does occur these systems, like the first category, can react too slowly for the particular application because of their reliance on the principle of incremental, gradual force correction. An example of this type of system is shown in U.S. Pat. No. 4,621,331, issued Nov. 4, 1986.
Previous fluid-powered slip correcting systems have mechanical shortcomings which further prevent them from reacting rapidly to the existence of slippage. For example, even though the system shown in the above-mentioned European Patent Publication No. 0443998A1 has been modified for commercial use by the addition of a pressurized fluid accumulator to improve its response to slippage, the system's fluid circuitry still detracts from its ability to respond quickly to slippage. This is due to several causes. One cause is the use of a relatively less responsive variably controlled relief valve to control gripping force, which valve wastefully depletes the accumulator's energy level by exhausting part of the accumulator fluid to the reservoir whenever the accumulator fluid is released to increase the grip. Other causes include the absence of continuously automatic recharging of the accumulator to maintain its energy level, necessary for repetitive responses to repetitive slip occurrences, and the exposure of the accumulator outlet to the remainder of the fluid circuitry which absorbs some of the energy of the accumulator's fluid output whenever the accumulator is activated.